1. Field of the Invention
The present invention relates to a magnetic recording/reproducing system such as a hard disk device and, more particularly, to a magnetic recording/reproducing system capable of substantially increasing the track density of a magnetic disk.
2. Description of the Related Art
In recent years, a magnetic recording/reproducing system such as a hard disk device has been popularly used as a random-accessible external storage device having a large capacity in the field of computers. As the magnetic recording/reproducing system is widely applied, an increase in storage capacity and an increase in recording density are strongly demanded. For this reason, in order to meet these demands, research and development have been performed in various fields.
In general, a hard disk device comprises a rotatable magnetic recording medium and a magnetic head member. The magnetic recording medium is constituted such that a plurality of magnetic disks each obtained by forming a magnetic layer on a nonmagnetic substrate are stacked and mounted on one rotating shaft. The magnetic head member is constituted such that recording/reproducing heads are arranged on an arm moved by an actuator. In this case, the recording/reproducing heads are arranged on the respective disk surfaces, and the heads are positioned such that the arms are moved by the actuator.
When information is to be recorded/reproduced by the hard disk device having the above structure, each of the heads is not brought into direct contact with a corresponding one of the disk surfaces rotated at high speed, and the head accesses a predetermined position of the disk surface while the head slightly floats from the disk surface. A signal is recorded by the head along concentric tracks on the disk surface, or the recorded signal is reproduced therefrom.
In the above hard disk device, in order to meet a demand of increasing a storage capacity, the following attempts have been performed. That is, the line recording density of a disk, i.e., a density in a direction of track length, is increased to increase a recording density, or a track width is decreased to increase a track density, thereby increasing a recording density.
In recent years, in order to increase a recording density, the research and development of contact recording in which a head floats at a very low level or brought into almost contact with a recording medium to record/reproduce a signal are energetically performed.
As a method of increasing a recording density, a perpendicular magnetic recording scheme was proposed in 1975. According to this perpendicular magnetic recording scheme, the demagnetization field of a magnetization transition portion is theoretically much smaller than that of a conventional longitudinal magnetic recording scheme in which an anisotropy is formed in a longitudinal direction. For this reason, the width of the magnetization transition is decreased, and high-density recording can be performed. In addition, according to the perpendicular magnetic recording scheme, a recording magnetic field having a more perpendicular direction can be obtained by a perpendicular magnetic recording head using a strip-like soft magnetic thin film. It is known that the perpendicular magnetic recording scheme is effective to increase a recording density.
In order to increase recording/reproduction efficiency to form a sharp magnetization transition, a vertical two-layered medium in which a soft magnetic backing layer is formed under a perpendicular anisotropic layer was proposed and has been developed. In this medium, the magnitude of a demagnetization field at the distal end of a head is decreased by magnetic interaction of the head and the soft magnetic backing layer, and a generated magnetic field having a large magnitude can be obtained. In reproduction, as in the recording, since the magnitude of demagnetization field at the distal end of the head is small, an effective magnetic permeability is increased, and magnetic fluxes from the medium are effectively converged on the head, thereby obtaining a signal having a large magnitude.
On the other hand, in order to improve sensitivity in signal reproduction, an active head such as an MR head using a magnetoresistance effect is popularly developed. The MR head is a head for converting a magnetic flux from a recording medium into an electrical signal using the nature in which the electric resistance of a soft magnetic material such as a permalloy is changed by an external magnetic field. Since this head performs reproduction such that a change in electric resistance of an MR element translates into a change in voltage, the reproduction sensitivity of the head is proportional to the magnitude of a sense current flowing in the soft magnetic material. For this reason, even when a relative speed of the head and the medium is low, a large output can be obtained, and a line recording density can be increased. In addition, as will be described in References 1 to 5, the large output from the MR head is effectively used to decrease a track width, so that a track density can be increased to a maximum of 17,000 TPI (Track Per Inches).
Reference 1: IEEE Transactions on Magnetics, Vol. 26, No. 5, 1689-1693 (1990); C. Tsang, M. Chen, T. Yogi and K. Ju, "Gigabit Density Recording Using Dual-element MR/Inductive Heads on Thin Film Disks"
Reference 2: IEEE Transactions on Magnetics, Vol. 26, No. 5, 2169-2171 (1990); R. Jensen, J. Mortelmans and R. Hauswitzer, "Demonstration of 500 Megabits per Square Inch with Digital Magnetic Recording"
Reference 3: IEEE Transactions on Magnetics, Vol. 26, No. 5, 2271-2276 (1990); T. Howell, D. McCown, T. Diola, Y. Tang, K. Hense and R. Gee, "Error Rate Performance of Experimental Gigabit per Square Inch Recording Components"
Reference 4: IEEE Transactions on Magnetics, Vol. 27, No. 6, 4678-4683 (1992); H. Takano, H. Futamoto, M. Suzuki, K. Shiiki and M. Kitada, "Submicron-Track Width Inductive/MR Composite Head"
Reference 5: IEEE Transactions on Magnetics, Vol. 27, No. 6, 5280-5285 (1992); M. Futamoto, F. Kugiya, M. Suzuki, H. Takano, Y. Matsuda, N. Inaba, Y. Miyamura, K. Akagi, T. Nakao, H. Sawaguchi, H. Fukuoka, T. Munemoto and T. Takagaki, "Investigation of 2 Gb/in2 Magnetic Recording at a Track Density of 17 kTPI"
In a magnetic recording/reproducing system such as a magnetic disk device, when a positioning error of a head for a track, i.e., a tracking error, is present, a so-called incomplete erasure phenomenon in which magnetization of an old recording signal is left by a tracking error in recording occurs. When the recorded signal is to be reproduced, a signal to be reproduced is reproduced, and, at the same time, an incompletely erased signal is reproduced as noise by a tracking error in reproduction. For this reason, a ratio of a signal to noise (SNR) is decreased.
As a technique for reducing the incomplete erasure or reducing an influence of the incomplete erasure, the following methods are known.
As a first method, a non-signal region (a non-signal region formed in the following manner is called an erase region) in which a substantially effective signal is not present is formed on each of both the side edges of a recording track using an erasing effect obtained by a magnetic field (called a side-fringe magnetic field) leaking from the gap of a recording head to the outside of the recording track, thereby reducing incomplete erasure. However, when a track density is increased by decreasing a track pitch, the side-fringe magnetic field may largely erase a signal on an adjacent track.
As the second method, a reproduction width Tr defined by a reproducing head is set to be smaller than the recording track width Tw defined by a recording head not to reproduce incomplete erasure (indicated by oblique lines) as much as possible. Although this can reduce noise generated by the incomplete erasure, since a reproduction output is decreased by a decrease in reproduction track width, an SNR cannot be sufficiently increased.